Evidence for a causal role of the renin-angiotensin system in nitrate tolerance

Background-We have previously shown that nitroglycerin (NTG) therapy increases vascular expression of endothelin 1 (ET-1) and stimulates vascular superoxide (O-2(.-))production via activation of NADH/NADPH oxidases. Both phenomena are stimulated by angiotensin II in vitro, and the renin-angiotensin system is activated during early nitrate therapy. We hypothesized that either angiotensin II or ET-1 may increase vascular O-2(.-) production during nitrate therapy. Methods and Results-In New Zealand White rabbits, 3 days of treatment with NTG patches increased plasma renin activity for the entire treatment period. After 24 hours of NTG treatment, angiotensin II type 1 (AT(1)) receptor expression and vascular ACE activity were significantly decreased, At this time, constrictions to angiotensin I and II were depressed, but there was no loss of NTG vasodilator potency. Within 3 days of continuous NTG treatment, relaxations to NTG were markedly blunted, This was associated with an increase in AT(1) receptor mRNA expression, a return of ACE activity back to baseline, and a marked increase in constrictions to angiotensin I and II despite continuously increased plasma renin activity. Tolerance was associated with a 2-fold increase in vascular O-2(.-), as estimated by lucigenin-enhanced chemiluminescence. Concomitant treatment with the AT(1) receptor antagonist losartan (5 to 25 mg.kg(-1).d(-1)) dose-dependently normalized vascular O-2(.-) and prevented tolerance to NTG and cross-tolerance to endogenous nitric oxide released by acetylcholine, The nonselective ET-1 receptor blocker bosentan (100 mg.kg(-1).d(-1)) had similar but less pronounced effects. Conclusions-The positive effects of AT(1) and ET-1 receptor blockade on tolerance and O-2(.-) production imply a pathophysiological role for angiotensin II and to some extent for ET-1 in the development of nitrate tolerance.